In Leptolinae the embryonic development culminates in a polyp, which
is usually formed by fixation of a planula (parenchymula), rarely by
fixation of an actinula. The planula may fix itself (1) by one end,
and then becomes the hydrocaulus and hydranth, while the hydrorhiza
grows out from the base; or (2) partly by one side and then gives rise
to the hydrorhiza as well as to the other parts of the polyp; or (3)
entirely by its side, and then forms a recumbent hydrorhiza from which
a polyp appears to be budded as an upgrowth.
In Trachylinae the development produces always a medusa, and there is
no polyp-stage. The medusa arises direct from the actinula-stage and
there is no entocodon formed, as in the budding described above.
_Life-cycles of the Hydromedusae._--The life-cycle of the Leptolinae
consists of an alternation of generations in which non-sexual
individuals, polyps, produce by budding sexual individuals, medusae,
which give rise by the sexual process to the non-sexual polyps again,
so completing the cycle. Hence the alternation is of the type termed
metagenesis. The Leptolinae are chiefly forms belonging to the inshore
fauna. The Trachylinae, on the other hand, are above all oceanic
forms, and have no polyp-stage, and hence there is typically no
alternation in their life-cycle. It is commonly assumed that the
Trachylinae are forms which have lost the alternation of generations
possessed by them ancestrally, through secondary simplification of the
life-cycle. Hence the Trachylinae are termed "hypogenetic" medusae to
contrast them with the metagenetic Leptolinae. The whole question has,
however, been argued at length by W. K. Brooks [4], who adduces strong
evidence for a contrary view, that is to say, for regarding the direct
type of development seen in Trachylinae as more primitive, and the
metagenesis seen in Leptolinae as a secondary complication introduced
into the life-cycle by the acquisition of _larval budding_. The polyp
is regarded, on this view, as a form phylogenetically older than the
medusa, in short, as nothing more than a sessile actinula. In
Trachylinae the polyp-stage is passed over, and is represented only by
the actinula as a transitory embryonic stage. In Leptolinae the
actinula becomes the sessile polyp which has acquired the power of
budding and producing individuals either of its own or of a higher
rank; it represents a persistent larval stage and remains in a
sexually immature condition as a neutral individual, sex being an
attribute only of the final stage in the development, namely the
medusa. The polyp of the Leptolinae has reached the limit of its
individual development and is incapable of becoming itself a medusa,
but only produces medusa-buds; hence a true alternation of generations
is produced. In Trachylinae also the beginnings of a similar
metagenesis can be found. Thus in _Cunina octonaria_, the ovum
develops into an actinula which buds daughter-actinulae; all of them,
both parent and offspring, develop into medusae, so that there is no
alternation of generations, but only larval multiplication. In _Cunina
parasitica_, however, the ovum develops into an actinula, which buds
actinulae as before, but only the daughter-actinulae develop into
medusae, while the original, parent-actinula dies off; here,
therefore, larval budding has led to a true alternation of
generations. In _Gonionemus_ the actinula becomes fixed and
polyp-like, and reproduces by budding, so that here also an
alternation of generations may occur. In the Leptolinae we must first
substitute polyp for actinula, and then a condition is found which can
be compared to the case of _Cunina parasitica_ or Gonionemus, if we
suppose that neither the parent-actinula (i.e. founder-polyp) nor its
offspring by budding (polyps of the colony) have the power of becoming
medusae, but only of producing medusae by budding. For further
arguments and illustrations the reader must be referred to Brooks's
most interesting memoir. The whole theory is one most intimately
connected with the question of the relation between polyp and medusa,
to be discussed presently. It will be seen elsewhere, however, that
whatever view may be held as to the origin of metagenesis in
Hydromedusae, in the case of Scyphomedusae (q.v.) no other view is
possible than that the alternation of generations is the direct result
of larval proliferation.
To complete our survey of life-cycles in the Hydromedusae it is
necessary to add a few words about the position of _Hydra_ and its
allies. If we accept the view that _Hydra_ is a true sexual polyp, and
that its gonads are not gonophores (i.e. medusa-buds) in the extreme
of degeneration, then it follows from Brooks's theory that _Hydra_
must be descended from an archaic form in which the medusan type of
organization had not yet been evolved. _Hydra_ must, in short, be a
living representative of the ancestor of which the actinula-stage is a
transient reminiscence in the development of higher forms. It may be
pointed out in this connexion that the fixation of _Hydra_ is only
temporary, and that the animal is able at all times to detach itself,
to move to a new situation, and to fix itself again. There is no
difficulty whatever in regarding _Hydra_ as bearing the same relation
to the actinula-stage of other Hydromedusae that a Rotifer bears to a
trochophore-larva or a fish to a tadpole.
_The Relation of Polyp and Medusa._--Many views have been put forward as
to the morphological relationship between the two types of person in the
Hydromedusae. For the most part, polyp and medusa have been regarded as
modifications of a common type, a view supported by the existence, among
Scyphomedusae (q.v.), of sessile polyp-like medusae (_Lucernaria_, &c.).
R. Leuckart in 1848 compared medusae in general terms to flattened
polyps. G. J. Allman [1] put forward a more detailed view, which was as
follows. In some polyps the tentacles are webbed at the base, and it was
supposed that a medusa was a polyp of this kind set free, the umbrella
being a greatly developed web or membrane extending between the
tentacles. A very different theory was enunciated by E. Metchnikoff. In
some hydroids the founder-polyp, developed from a planula after
fixation, throws out numerous outgrowths from the base to form the
hydrorhiza; these outgrowths may be radially arranged so as to form by
contact or coalescence a flat plate. Mechnikov considered the plate thus
formed at the base of the polyp as equivalent to the umbrella, and the
body of the polyp as equivalent to the manubrium, of the medusa; on this
view the marginal tentacles almost invariably present in medusae are new
formations, and the tentacles of the polyp are represented in the medusa
by the oral arms which may occur round the mouth, and which sometimes,
e.g. in _Margelidae_, have the appearance and structure of tentacles.
Apart from the weighty arguments which the development furnishes against
the theories of Allman and Mechnikov, it may be pointed out that neither
hypothesis gives a satisfactory explanation of a structure universally
present in medusae of whatever class, namely the endoderm-lamella,
discovered by the brothers O. and R. Hertwig. It would be necessary to
regard this structure as a secondary extension of the endoderm in the
tentacle-web, on Allman's theory, or between the outgrowths of the
hydrorhiza, on Mechnikov's hypothesis. The development, on the contrary,
shows unequivocally that the endoderm-lamella arises as a local
coalescence of the endodermal linings of a primitively extensive gastral
space.
The question is one intimately connected with the view taken as to the
nature and individuality of polyp, medusa and gonophore respectively. On
this point the following theories have been put forward.
1. The theory that the medusa is simply an organ, which has become
detached and has acquired a certain degree of independence, like the
well-known instance of the hectocotyle of the cuttle-fish. On this
view, put forward by E. van Beneden and T. H. Huxley, the sporosac is
the starting-point of an evolution leading up through the various
types of gonophores to the free medusa as the culminating point of a
phyletic series. The evidence against this view may be classed under
two heads: first, comparative evidence; hydroids very different in
their structural characters and widely separate in the systematic
classification of these organisms may produce medusae very similar, at
least so far as the essential features of medusan organization are
concerned; on the other hydroids closely allied, perhaps almost
indistinguishable, may produce gonophores in the one case, medusae in
the other; for example, _Hydractinia_ (gonophores) and _Podocoryne_
(medusae), _Tubularia_ (gonophores) and _Ectopleura_ (medusae),
_Coryne_ (gonophores) and _Syncoryne_ (medusae), and so on. If it is
assumed that all these genera bore gonophores ancestrally, then medusa
of similar type must have been evolved quite independently in a great
number of cases. Secondly, there is the evidence from the development,
namely, the presence of the entocodon in the medusa-bud, a structure
which, as explained above, can only be accounted for satisfactorily by
derivation from a medusan type of organization. Hence it may be
concluded that the gonophores are degenerate medusae, and not that the
medusae are highly elaborated gonophores, as the organ-theory
requires.
2. The theory that the medusa is an independent individual, fully
equivalent to the polyp in this respect, is now universally accepted
as being supported by all the facts of comparative morphology and
development. The question still remains open, however, which of the
two types of person may be regarded as the most primitive, the most
ancient in the race-history of the Hydromedusae. F. M. Balfour put
forward the view that the polyp was the more primitive type, and that
the medusa is a special modification of the polyp for reproductive
purposes, the result of division of labour in a polyp-colony, whereby
special reproductive persons become detached and acquire organs of
locomotion for spreading the species. W. K. Brooks, on the other hand,
as stated above, regards the medusa as the older type and looks upon
both polyp and medusa, in the Hydromedusae, as derived from a
free-swimming or floating actinula, the polyp being thus merely a
fixed nutritive stage, possessing secondarily acquired powers of
multiplication by budding.
The Hertwigs when they discovered the endoderm-lamella showed on
morphological grounds that polyp and medusa are independent types,
each produced by modification in different directions of a more
primitive type represented in development by the actinula-stage. If a
polyp, such as _Hydra_, be regarded simply as a sessile actinula, we
must certainly consider the polyp to be the older type, and it may be
pointed out that in the Anthozoa only polyp-individuals occur. This
must not be taken to mean, however, that the medusa is derived from a
sessile polyp; it must be regarded as a direct modification of the
more ancient free actinula form, without primitively any intervening
polyp-stage, such as has been introduced secondarily into the
development of the Leptolinae and represents a revival, so to speak,
of an ancestral form or larval stage, which has taken on a special
role in the economy of the species.